1. Technical Field
The present invention relates to a surface acoustic wave resonator, and a surface acoustic wave oscillator and an electronic device using the surface acoustic wave resonator.
2. Related Art
In the past, surface acoustic wave (SAW) filters have been widely used as band-pass filters of mobile communication devices. As such conventional surface acoustic wave filters, there can be cited surface-wave resonator filters, transversal filters, and so on.
For example, as one of the conventional surface-wave resonator filters, there is a filter using an ST-cut 0° X propagation quartz crystal substrate with preferable temperature characteristics, and having an interdigital transducer (IDT), a reflector, and so on formed on the quartz crystal substrate using an electrode material made of Al so as to use the Rayleigh wave caused by the excitation of the IDT.
Further, as another of the surface-wave resonator filters, there is a filter using an ST-cut 90° X propagation quartz crystal substrate, and having an IDT, a reflector, and so on formed on the quartz crystal substrate using an electrode material made of Ta, W, Au, and so on so as to use the SH wave caused by the excitation of the IDT.
However, in the former surface-wave resonator filter having the electrode made of Al formed on the ST-cut 0° X propagation quartz crystal substrate, since the Rayleigh wave is used, there arises the following problems.
1. Since the reflectivity coefficient is small, and a number of reflector fingers are required in the device using the reflector such as a surface-wave resonator filter, downsizing is hindered.
2. Since the electromechanical coupling coefficient is small, the loss is significant.
On the other hand, the latter surface-wave resonator filter having the electrode formed on the ST-cut 90° X propagation quartz crystal substrate using the metal having a large mass load such as Ta, W, or Au has features that the electromechanical coupling coefficient is large since the SH wave is used, and that downsizing of the device can be achieved since the reflectivity coefficient is also large.
However, if the electrode is formed using the metal having a large mass load such as Ta, W, or Au, there arises a problem that the variation in the central frequency due to the variation in the width and thickness of the electrode becomes large to thereby raise the defective fraction. In other words, the larger the mass load of the material used for forming the electrode becomes, the steeper the variation in the acoustic velocity with respect to the film thickness becomes compared to Al with a small mass load, and there arises a problem that the variation in the central frequency becomes larger even with the same level of variation in the width and the thickness of the electrode as in the case of using Al with a small mass load.
In order for solving such problems, in JP-A-2002-330051 there is a description that the normalized film thickness (H/λ) of the fingers of the IDT formed of a metal film made of Al on the quartz crystal substrate for generating the SH wave is optimized, thereby obtaining a surface acoustic wave filter having a large electromechanical coupling coefficient with a small loss, and capable of reducing the influence of the film thickness of the electrode exerted on the frequency.
However, even in the document mentioned above, there is a problem that if the film thickness becomes large, a variation occurs in the film thickness to cause a variation in frequency, and thus degrading the yield to thereby raise the cost. Further, there is also a problem that the frequency fluctuation due to the heat strain with respect to the quartz crystal substrate and the temporal change becomes marked. Still further, the improvement in the electromechanical coupling coefficient has limitations.